The use of the progressive form underlines the fact that "I" should thank "you" right now, at this very moment. The simple form sounds equally good to my Helsinki English ear, though. New Zealanders and Memphians may disagree, of course!
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BamtoriPlease would anyone explain why the progressive form was used in here?:
"I should be thanking you! It should be the other way around."
Also, this one, "Shouldn't be taking so long." . The situation is that a guy is waiting for something to happen. Thanks